Well, I have given class levels to animals before, just add the Spellwarped template and you are free to go. Monkeys do awesome rogues, and Ravens are itneresting druids.
I'm totally gonna make a fiendish sentry ooze gelatinous cube Swordsage to encounter my party. Sentry ooze gives it an Int score, which fiendish sets to 3. With an Int of 3, he can now gain class levels. Go go Shadow Jaunt Ooze! Five Pseudopod Creeping Enervation Strike!
The cube gets done...
I believe the thinking on class levels is "So long as a creature is intelligent enough to want to train itself to be better or towards a goal, it should be able to take class levels." Which is typically the Int 3 cutoff point.